A standard technique for producing containers is blow molding (optionally combined with a stretching). This technique consists in introducing the blank (i.e., a preform or an intermediate container that has undergone a preforming operation), previously heated to a temperature that is higher than the glass transition temperature of the material (about 80° in the case of PET), in a mold provided with a wall that defines a cavity having the shape of the container, and in injecting into the blank, through the neck, a fluid, such as a gas (generally air), under pressure to press the material against the wall of the mold.
Ordinarily, this injection of fluid is performed in two steps, namely a first step (known as pre-blow molding) at low pressure (generally from about 5 to 7 bar), followed by a second step (known as blow molding) at high pressure (generally from about 20 to 30 bar).
For hot fill applications, requiring that the container bear greater thermal stresses, it is known to make an overstretched base, intended either to absorb the deformation of the container during a hot fill, or to better withstand the thermal stresses.
To obtain such an overstretched base, normally a molding unit is used that is equipped with a mobile mold base that is initially retracted and that is deployed during forming to push back the material in the area of the base, as is illustrated in the U.S. Pat. No. 6,277,321 (SCHMALBACH-LUBECA).
This technique, currently known as “boxing,” is rather difficult to use. Actually, a compromise must be found between the desire to stretch the material beyond its final form so as to increase the taking of the shape of the base and the necessity of preventing the material from setting before having acquired its final form. Although this concern is explicitly mentioned in the above-mentioned document, it, however, remains vague as to solutions that make it possible to remedy it, merely suggesting that it is preferable to limit the delay between the pre-blow-molding phase and the raising of the mold base.
In practice, the manner in which the boxing is conducted has direct consequences on the quality and the performances of the base of the container.
The theoretical modeling of the taking of shape of the base, useful for configuring the machine that is expected to make it possible to obtain containers that conform to specifications, is generally based on the hypothesis of a raising of the mold base at a constant speed (linear model).
The linear model does not, however, take reality correctly into account. Actually, in view of the pressure variations within the container, the resistance exhibited by it to the raising of the mold base during boxing is also variable, which causes a variation of the speed of movement of the mold base.
Under these conditions, it is found that not only does the mold base not achieve its travel at the time anticipated by the theoretical model but also that differences from one mold to the next (and therefore from one container to the next) appear, making it difficult to obtain identical containers that conform to the same specifications.
Furthermore, it will be noted that, given the initial lowered position of the mold base, there is a risk that the material that is overstretched radially beyond the limits of the base of the final container will be pinched (and as the case may be, perforated) during the raising of the mold base.
The reasons that have just been mentioned provide an incentive, with good reason, to anticipate the boxing relative to the blow molding, i.e., to perform the raising of the mold base before injecting the fluid into the blank at high pressure. Moreover, this is what the document U.S. Pat. No. 6,277,321 cited above recommends—without, however, completely justifying it.
To date, due to a lack of direct observation means of the blank during forming, the checking of the quality of the container obtained, and the validation (or the modification) of the parameters programmed into the machine (particularly pressure and blow-molding flow rate, time of ordering movement of the mold base), are performed by operators who are sufficiently qualified and experienced to visually and manually evaluate the quality of the containers.
However, the human checking, although necessary, is nevertheless too long and subjective to make possible a programming that is reliable, rapid and especially that can be extended to the entire set of machines. Since to date the settings are made by guesswork, it is still often necessary, in order to prevent an accumulation of non-conforming containers, to halt the production line.